1. Field of the Invention
The present invention relates to a sheet post-processing apparatus provided with a function of saddle stitch operation or folding processing, and particularly to a sheet post-processing apparatus capable of realizing high accuracy folding processing.
2. Description of the Related Art
There is developed a sheet post-processing apparatus having a bookbinding function for binding plural sheets, on which images have been formed, by staples. Binding methods performed in the sheet post-processing apparatus as stated above include end stitch of stitching a sheet bundle at one place or plural places on the end side thereof, and saddle stitch of stitching a sheet bundle at plural places of the center part thereof. Some apparatus to perform the saddle stitch has a saddle stitch folding function to fold the center part of the stitched sheet bundle to form something like a weekly magazine.
In the saddle stitch folding function to fold the center part of the stitched sheet bundle to form something like the weekly magazine, folding means at the center part includes a blade and a pair of folding rollers (for example, see JP-A-2005-8418).
This is such that the folding means at the center part of the sheet bundle pushes the center part of the transported sheet bundle into the facing part of the pair of folding rollers by the blade, and the folding processing is performed to the sheet bundle by pressurization due to the rotation of the folding rollers.
The sheet bundle subjected to the folding processing is discharged from the folding rollers, is transported along a transport path, and is accumulated on a saddle tray.
Since it is necessary that the folding rollers fold the sheet bundle with high accuracy, low speed rotation at a constant speed is generally used. However, although the folding accuracy of the sheet bundle varies according to the sheet type, such as thick paper, waterproof paper or color print paper, or the sheet size, the folding accuracy corresponding to each of them can not be obtained by the low speed rotation at the constant speed.
Besides, in the case where high speed rotation at a constant speed is performed, the body of folding of the sheet bundle is not satisfactorily obtained, and there occurs a disadvantage that the sheet bundle is expanded after being folded.
Besides, also with respect to the drive speed of the blade to push the sheet bundle into between the folding rollers, a constant speed is used. In this case, when the blade is driven at a high speed, even if an adjustment is made so that the blade comes in contact with the center of the sheet bundle, a shift occurs immediately after the instant when the blade comes in contact with the sheet bundle.
On the other hand, in the case where the blade is driven at a low speed, a load is applied to the blade at the folding, and a very high force is required. Thus, a powerful driving source is required.